Electrodeless high intensity discharge (EHID) lamps have been described extensively in the prior art. In general, EHID lamps include an electrodeless lamp capsule containing a volatilizable fill material and a starting gas. The lamp capsule is mounted in a fixture which is designed for coupling high frequency power to the lamp capsule. The high frequency produces a light-emitting plasma discharge within the lamp capsule. Recent advances in the application of high frequency power to lamp capsules operating in the tens of watts range are disclosed in U.S. Pat. No. 5,070,277 issued Dec. 3, 1991 to Lapatovich; U.S. Pat. No. 5,113,121 issued May 12, 1992 to Lapatovich et al; U.S. Pat. No. 5,130,612 issued Jul. 14, 1992 to Lapatovich et al; U.S. Pat. No. 5,144,206 issued Sep. 1, 1992 to Butler et al; and U.S. Pat. No. 5,241,246 issued Aug. 31, 1993 to Lapatovich et al. As a result, compact EHID lamps and associated applicators have become practical.
The above patents disclose small, cylindrical lamp capsules wherein high frequency energy is coupled to opposite ends of the lamp capsule with a 180.degree. phase shift. The applied electric field is generally colinear with the axis of the lamp capsule and produces a substantially linear discharge within the lamp capsule. The fixture for coupling high frequency energy to the lamp capsule typically includes a planar transmission line, such as a microstrip transmission line, with electric field applicators, such as helices, cups or loops, positioned at opposite ends of the lamp capsule. The microstrip transmission line couples high frequency power to the electric field applicators with a 180.degree. phase shift. The lamp capsule is typically positioned in a gap in the substrate of the microstrip transmission line and is spaced above the plane of the substrate by a few millimeters, so the axis of the lamp capsule is colinear with the axes of the field applicators.
A well-optimized applicator should exhibit several characteristics. It should transfer power from the power source to the lamp with the highest possible efficiency. In particular, resistive heating in the applicator, microwave radiation which produces electromagnetic interference, and power reflected back toward the power source must be minimized. The applicator should be small and light; it should not block light from the lamp; and its operation should not be substantially perturbed by the proximity of metal or dielectric structures.
Anticipated applications of EHID lamps require mounting the lamp in a focusing reflector or similar optical system. In the past, this has usually required cutting a slot in the reflector in order to accommodate the circuit board of the planar applicator. The slot is often difficult and expensive to make. The slot wastes light and may create a dark spot in the outgoing beam pattern. In many cases, the optical design cannot be changed, or changes such as a slot would weaken the optical assembly or make it susceptible to environmental exposure.
Several types of power applicators for energizing EHID lamps are known in the prior art. For large EHID lamps ranging in size from a few millimeters in diameter to 25 or 30 millimeters in diameter, coupling of power using a cylindrical cavity is taught by MacLennan et al in paper P-73, SID 93 Digest, pages 716-719, 1993 and by Lynch et al in U.S. Pat. No. 4,954,755 issued Sep. 4, 1990. Spherical lamps are rotated about the stem supporting the lamp. For small cylindrical lamps, close coupling planar applicators made from printed circuit substrate material are disclosed by Lapatovich et al in U.S. Pat. No. 5,280,217 issued Jan. 18, 1994. For small spherical lamps of about 2-10 millimeters in diameter, a planar applicator fabricated from printed circuit board material using a rotating electric field is disclosed by Lapatovich et al in U.S. Pat. No. 5,498,928 issued Mar. 12, 1996. A hybrid applicator cavity/optical element is disclosed by Simpson et al in U.S. Pat. No. 4,887,192 issued Dec. 12, 1989. Electrodeless light sources, wherein an electrodeless lamp is mounted in a reflector, are disclosed in U.S. Pat. No. 4,749,915 issued Jun. 7, 1988 to Lynch et al; U.S. Pat. No. 5,299,100 issued Mar. 29, 1994 to Bellows et al; and U.S. Pat. No. 5,448,135 issued Sep. 5, 1995 to Simpson.
The cavity approach results in a large cylindrical mesh shell which does not mate well with small optical collectors, such as an automobile headlamp. The planar applicators do mate with the optical system, but their circuit boards block considerable light, and the reflectors must be slotted as described above. The rotating field applicator requires that the collector be formed in two sections and aligned around the lamp and applicator.
A variety of large coaxial termination fixtures with mesh covers were developed for high wattage electrodeless lamps for the motion picture industry as disclosed by Haugsjaa et al in U.S. Pat. No. 3,942,058 issued Mar. 2, 1976, U.S. Pat. No. 3,942,068 issued Mar. 2, 1976, U.S. Pat. No. 3,943,403 issued Mar. 9, 1976, U.S. Pat. No. 3,995,195 issued Nov. 30, 1976 and U.S. Pat. No. 4,001,632 issued Jan. 4, 1977. Coaxial termination fixtures for electrodeless lamps are also disclosed in U.S. Pat. No. 4,185,228 issued Jan. 22, 1980 to Regan; U.S. Pat. No. 4,189,661 issued Feb. 19, 1980 to Haugsjaa et al; U.S. Pat. No. 4,223,250 issued Sep. 16, 1980 to Kramer et al; U.S. Pat. No. 4,247,800 issued Jan. 27, 1981 to Proud et al; and U.S. Pat. No. 4,266,162 issued May 5, 1981 to McNeill et al. None of these fixtures are well optimized in terms of manufacturability or efficient operation of small lamps. The meshes are not shaped so as to guide the electric fields through the lamp, and they must be attached to the body of the applicator with a large number of mechanically and electrically sound connections. The variable impedance transmission lines used to match impedances are excessively long and lossy.
The aforementioned U.S. Pat. No. 3,942,058 describes the concept of field shaping, but shows devices which are unlikely to work well except for spherical or very short lamps. The outer conductor is not contoured for field shaping.
Thus, there exists a need for power applicators for EHID lamps which fit through the small hole in the rear of a typical reflector, and which can be integrated into existing optical systems effectively and inexpensively.